serial: serial_stm32: Enable uart FIFO for STM32F7xx SoCs

serial: serial_stm32: Enable uart FIFO for STM32F7xx SoCs

Since commit 7b3b74d32127 ("serial: serial_stm32: Enable overrun")
on STM32F7xx based boards, the first lines of serial output are
missing during boot (we no more see the U-Boot release version,
board model and DRAM size).

By enabling the uart FIFO on STM32F7, the complete U-boot log
can be sent correctly.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>